The term “gilt” or “gilt gloss” refers to dials created by a galvanic coating process, which appears on GMTs until 1967. Galvanization refers to a method of coating metals with another substance through a binding chemical reaction. Source
In a multi-step process, the dials were first stamped with a clear coating that delineated the text and markers. This would prevent the galvanically applied black paint from binding to those parts of the dial. A clear lacquer was then applied, giving the dial a glossy finish. After lacquering, further text (such as underlines or depth ratings) could be added. Finally, the lumed hour markers were applied and the dial was complete. Source
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Top Picture Credit: Bexsonn
Rolex GMT Master 1675 